linux-ext4 - Re: [PATCH V5 0/8] mm/slab: reduce slab accounting memory overhead by allocating slabobj

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Message-ID: <14a0f149-5d22-4a3b-9cbf-3336d0783e9d@suse.cz>
Date: Wed, 7 Jan 2026 18:43:30 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Harry Yoo <harry.yoo@...cle.com>, akpm@...ux-foundation.org
Cc: andreyknvl@...il.com, cl@...two.org, dvyukov@...gle.com,
 glider@...gle.com, hannes@...xchg.org, linux-mm@...ck.org,
 mhocko@...nel.org, muchun.song@...ux.dev, rientjes@...gle.com,
 roman.gushchin@...ux.dev, ryabinin.a.a@...il.com, shakeel.butt@...ux.dev,
 surenb@...gle.com, vincenzo.frascino@....com, yeoreum.yun@....com,
 tytso@....edu, adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
 linux-kernel@...r.kernel.org, cgroups@...r.kernel.org, hao.li@...ux.dev
Subject: Re: [PATCH V5 0/8] mm/slab: reduce slab accounting memory overhead by
 allocating slabobj_ext metadata within unsed slab space

On 1/5/26 09:02, Harry Yoo wrote:
> Happy new year!
> 
> V4: https://lore.kernel.org/linux-mm/20251027122847.320924-1-harry.yoo@oracle.com
> V4 -> V5:
> - Patch 4: Fixed returning false when the return type is unsigned long
> - Patch 7: Fixed incorrect calculation of slabobj_ext offset (Thanks Hao!)

Besides the stuff pointed out the rest seemed ok to me. Can you resend with
those addressed, and rebased on slab.git slab/for-7.0/obj_metadata to avoid
a conflict in patch 8/8 with Hao's comment update patch there? I will add
the series on top there then. Thanks!

> When CONFIG_MEMCG and CONFIG_MEM_ALLOC_PROFILING are enabled,
> the kernel allocates two pointers per object: one for the memory cgroup
> (actually, obj_cgroup) to which it belongs, and another for the code
> location that requested the allocation.
> 
> In two special cases, this overhead can be eliminated by allocating
> slabobj_ext metadata from unused space within a slab:
> 
>   Case 1. The "leftover" space after the last slab object is larger than
>           the size of an array of slabobj_ext.
> 
>   Case 2. The per-object alignment padding is larger than
>           sizeof(struct slabobj_ext).
> 
> For these two cases, one or two pointers can be saved per slab object.
> Examples: ext4 inode cache (case 1) and xfs inode cache (case 2).
> That's approximately 0.7-0.8% (memcg) or 1.5-1.6% (memcg + mem profiling)
> of the total inode cache size.
> 
> Implementing case 2 is not straightforward, because the existing code
> assumes that slab->obj_exts is an array of slabobj_ext, while case 2
> breaks the assumption.
> 
> As suggested by Vlastimil, abstract access to individual slabobj_ext
> metadata via a new helper named slab_obj_ext():
> 
> static inline struct slabobj_ext *slab_obj_ext(struct slab *slab,
>                                                unsigned long obj_exts,
>                                                unsigned int index)
> {
>         return (struct slabobj_ext *)(obj_exts + slab_get_stride(slab) * index);
> } 
> 
> In the normal case (including case 1), slab->obj_exts points to an array
> of slabobj_ext, and the stride is sizeof(struct slabobj_ext).
> 
> In case 2, the stride is s->size and
> slab->obj_exts = slab_address(slab) + s->red_left_pad + (offset of slabobj_ext)
> 
> With this approach, the memcg charging fastpath doesn't need to care the
> storage method of slabobj_ext.
> 
> Harry Yoo (8):
>   mm/slab: use unsigned long for orig_size to ensure proper metadata
>     align
>   mm/slab: allow specifying free pointer offset when using constructor
>   ext4: specify the free pointer offset for ext4_inode_cache
>   mm/slab: abstract slabobj_ext access via new slab_obj_ext() helper
>   mm/slab: use stride to access slabobj_ext
>   mm/memcontrol,alloc_tag: handle slabobj_ext access under KASAN poison
>   mm/slab: save memory by allocating slabobj_ext array from leftover
>   mm/slab: place slabobj_ext metadata in unused space within s->size
> 
>  fs/ext4/super.c      |  20 ++-
>  include/linux/slab.h |  39 +++--
>  mm/memcontrol.c      |  31 +++-
>  mm/slab.h            | 120 ++++++++++++++-
>  mm/slab_common.c     |   8 +-
>  mm/slub.c            | 345 +++++++++++++++++++++++++++++++++++--------
>  6 files changed, 466 insertions(+), 97 deletions(-)
>